An optical communication system including a trunk line is desired to have a larger capacity because traffic has been increased in the Internet. An optical receiver that may transmit a signal of 100 [Gbit/s] or more per wavelength. If a bit rate per wavelength is increased, deterioration of a signal quality due to Optical Signal Noise Ratio (OSNR), chromatic dispersion, polarization mode dispersion or nonlinear effect in the transmission path is increased.
Therefore, a digital coherent receiving method has been proposed in recent years because of OSNR benefit as well as the potential to compensate for all linear transmission impairment. a direct detecting method which is conventional way for the optical communication system allocates ON/OFF of a light intensity to a binary signal. On the other hand, the digital coherent receiving method extracts the intensity and phase of received light by using the coherent receiving method. The extracted intensity and phase information are quantized by an Analog/Digital (A/D) convertor and are demodulated by a digital signal processor.
There is an advantage of which the digital coherent receiver can synchronize a frequency and a phase between a transmission light source and a local oscillator light source by using digital signal processing. Accordingly, optical Phase Locked Loop (PLL), which is difficult to be realized, is not needed.
Typically available tunable laser have end of life frequency accuracy of several [GHz]. A frequency difference (frequency offset) between a light source of a transmitter and a Local Oscillator (LO) of a receiver may occur within a range of frequency accuracy.
For example, reception of a Quadrature Phase Shift Keying (QPSK) signal will be described. If the frequency offset is small enough, only static rotation caused by a phase shift is observed in the signal constellation observed for a short time. On the other hand, if the frequency offset is large, a constellation is rotated by the frequency offset during an observation time by the frequency offset. As a result, identification of a QPSK signal becomes difficult. Therefore, compensation of the frequency offset is used. Hereinafter, as well as the frequency difference between the transmission optical source and the LO optical source, a phase rotation caused by a frequency offset will be referred to as a frequency offset.
To compensate for the frequency offset, for example, the frequency offset may be estimated by the frequency offset estimation circuit in digital signal processing, and the constellation is inverse rotated in a digital circuit by the estimated frequency offset. For example, m-th power method is well known technique for frequency offset estimation, which removes a coding component by raising the MPSK signal of an m value to the m-th power and extracts the component of the frequency offset (see, for example, Non-Patent Document 1). Furthermore, there is a known technique for expanding an estimation range of the frequency offset estimator by using performing temporary determination of a signal to remove the coding component (see, for example, Non-Patent Document 2).    [Non-Patent Document 1] A. Leven et al. “Frequency Estimation in Intradyne Reception”, IEEE Photonics Technology Letters, Vol. 19, No. 6, p. 366-368, March, 2007    [Non-Patent Document 2] H. Nakashima et al. “Novel Wide-range Frequency Offset Compensator Demonstrated with Real-time Digital Coherent Receiver”, 34th European Conference and Exhibition on Optical Communication (ECOC2008), Mo.3.D.4, p. 1-2, September, 2008
In the above-described prior art, however, there is limitation of a pull-in range in an estimation circuit of the frequency offset, so that the frequency offset outside the pull-in range may not be accurately estimated. As a result, the large frequency offset may not be accurately compensated, and the reception quality is deteriorated. To solve the above-described problem, differential decoding may be used. However, this may cause a problem that the OSNR property is deteriorated.
The disclosed digital coherent receiver and receiving method are used to solve the above-described problems and improve the reception quality.